The present invention generally relates to a photoelectron mask for photo cathode image projection and a photo cathode image projection method using the same. More particularly the present invention is directed to the development of photo cathode materials, and a photo cathode image projection method which uses photoelectron masks in which the developed photo cathode materials are used.
As a fine pattern technique of producing a semiconductor integrated circuit such as a very large scale integrated circuit (VLSI), there is an exposure technique in which a fine pattern is exposed on a wafer. An ultraviolet-ray exposure method is used as lithography technology for a long time. However, there is a limit on the width of a fine pattern because a wavelength of light which can be used in the ultraviolet-ray exposure method is limited to approximately 4000 .ANG.. Therefore, active research is being made on new exposure techniques as electron beam lithography, X-ray lithography, and photo cathode image projection lithography.
In electron beam lithography, an electron beam shaped into a spot or an electron beam having a rectangular cross section is deflected and irradiated on a wafer mounted on a stage, which is moved during irradiation. Therefore, electron beam lithography needs a column system for focusing, shaping and deflecting the electron beam emitted from an electron source, a stage system for supporting the wafer and changing the pattern image transferred position thereon, and a control system for controlling the column system and the stage system. Electron beam lithography is capable of providing high resolution. However a pattern is drawn with a single stroke of the electron beam based on enormous pattern data. For this reason, exposure of the wafer take a long time. Additionally, throughput which is defined as the number of wafers that can be exposed per hour, is low. It follows that electron beam lithography is not suitable for mass production of wafers.
X-ray lithography is proximity printing in which there is used a large-scale X-ray source having a power of 10 to 50 kW and an X-ray beam having a wavelength of 1 to 10 .ANG.. Therefore, in addition to the above-mentioned X-ray source, the X-ray exposure method needs a mask, and an aligner which can support the wafer and position the wafer and mask with a high positioning accuracy. From this viewpoint, the X-ray exposure method is similar to a conventional photoexposure process. However, the X-ray exposure method has the following disadvantages. First, a large-scale X-ray source is required. Second a material used for fabricating a mask must be carefully noted in view of the relationship between the wavelength of the X-ray source and the X-ray absorption rate. Third, an increased diameter of the wafer may cause deformation such as warpage and bending in the mask. As a result, a gap deformation, and a fine X-ray image cannot be formed on the wafer. Fourth, it is difficult to obtain a strong X-ray intensity, and throughput is poor. Synchrotron radiation is proposed as the X-ray source. However, the use of synchrotron radiation increases the size of an apparatus, and is therefore very expensive. Thus the use of the synchrotron radiation is not suitable for practical use.
Recently, there has been considerable activity in the research and development of photo cathode image projection. The photo cathode image projection method can provide high throughput and high resolution. Two different types of photo cathode image projection are known. In the first type of photo cathode image projection, a metal-insulator-semiconductor structure is used as a mask. Light is projected onto a semiconductor layer which then produces photoelectrons. The produced photoelectrons pass through an insulator such as a silicon dioxide layer. Then the photoelectrons pass through a metal film such as a silver film, and are emitted. It is noted that in the first type of photo cathode image projection, the metal film itself does not produce electrons. As described above, electrons are produced in the semiconductor layer.
On the other hand, the second type of photo cathode image projection uses a mask in which a photo cathode material is used. When light having a work function higher than a potential barrier of a photo cathode material is projected thereon, the photo cathode material produces photoelectrons. Generally, a pattern of a non-transparent material is formed on a transparent substrate. A film made of a photo cathode material is formed so as to cover the pattern. Light is projected onto a rear source of the substrate opposite to the surface on which the pattern is formed. Photoelectrons are emitted from portions of the photo cathode material film which do not overlie the pattern. The emitted photoelectrons are accelerated and focused on the wafer by the function of the magnetic and electric fields. Then the pattern image is exposed on the wafer. The present invention is concerned with the second type of photo cathode image projection.
It is desired that photo cathode materials be capable of stably emitting strong photoelectrons for a long time. In addition, it is desired that there exists a material capable of providing large contrast to photo cathode materials. Moreover, it is desired that photo cathode materials are stable in the air, and are easily handled to form a mask.
Conventionally, hydroiodic cesium (CsI) is proposed as a photo cathode material. A photo cathode is formed by depositing hydroiodic cesium on the entire surface of the substrate by evaporation under vacuum. However, hydroiodic cesium has a disadvantage in which energy at the basic absorption end of light is high. This means that the work function of hydroiodic cesium is high. The work function is defined as the height of the potential barrier. Photoelectrons excited by the projection of light must go over the potential barrier so that electrons are emitted from the hydroiodic cesium. Additionally, hydroiodic cesium absorbs water with ease and is therefore unstable. As a result, the pattern formed on the substrate is quickly damage. In practical use, the exposure can be repetitively carried out only 20 to 50 times.
Galium arsenic is also proposed as one of photo cathode materials. In practical use, a cesium thin-film is often formed on a galium arsenic film. The cesium thin-film functions to decrease the work function. Therefore, the above film structure can provide high quantum efficiency. However, there is a problem in which foreign materials are easily deposited on the cesium thin-film and thus, the life time of the mask is very short.
Silver oxide is also proposed as one of photo cathode materials. Silver oxide has a high quantum efficiency. However, it has a disadvantage in which there is no material capable of providing good contrast with respect to silver oxide.
The present inventors have proposed cesium-added silver as a photo cathode material in U.S. Pat. application Ser. No. 086,510. In the proposal, a cesium film is formed on a patterned silver film under vacuum. Cesium may be substituted with other alkaline metals or alkaline-earth metals. Silver used together with cesium is more stable than hydroiodic cesium and has the lifetime longer than that of hydroiodic cesium. However, cesium is liable to leave the silver film due to the projection of light. A decrease of cesium on the silver film decreases the amount of photoelectrons emitted from the silver pattern. Therefore, it is necessary to frequently activate the mask.
The following document proposes to use palladium as a photo cathode material: An-Electron-Image Projector With Automatic Alignment J. P. Scott., IEEE Trans. on Electron Devices, ED-22 409-413, 1975. The above document indicates that hydroiodic cesium is better than palladium from various viewpoints.
It can be seen from the above description that with conventional materials, it is impossible to satisfy all the factors desired from photo cathode materials. It is particularly noted that the aforementioned various problems arise from the presence of alkaline metals or alkaline-earth metals.